Portable communication devices, for example, cellular telephones, are often required to operate over a number of different communication bands. These so-called “multi-band” communication devices use one or more instances of transmit and receive circuitry to generate and amplify the transmit and receive signals. These communication devices may employ a single antenna to transmit and receive the signals over the various communication bands or, in more recent devices, may use multiple antennas, each communicating over various communication bands, such as an antenna for high frequency bands, and another for low and middle frequency bands.
Each antenna in such communication devices is typically connected to the transmit and receive circuitry through switching circuitry, such as a duplexer or a diplexer, or through an isolated switch element, sometimes referred to as a “transmit/receive switch” or an “antenna switch.” The switching circuitry or the isolated switch element must effectively isolate the transmit signal from the receive signal. Isolating the transmit signal from the receive signal becomes more problematic in a multiple band communications device where the transmit frequency of one communication band might overlap with the receive frequency of a different communication band.